1. Field of Invention
This invention relates to a system for passively integrating light from a diode pumped waveguide. Specifically, this invention uses an organic light emitting diode (OLED) to optically pump an image bar.
2. Description of Related Art
Organic light emitting diodes (OLEDs) are known and used conventionally as light emitters in image forming devices, such as displays. However, OLEDs are often unable to provide enough light to a photoreceptor of an electrophotographic print engine to allow the image forming engine to operate at a sufficiently high process speed. FIG. 1 illustrates a conventional OLED. In devices that emit light normal to their substrate, such as that illustrated in FIG. 1, a large percentage of the light generated by the OLED does not reach the photoreceptor. This large percentage of light loss in conventional OLEDs is a result of a number of lossy interactions. Furthermore, the amount of current input to the OLED is limited because the materials used to form the OLED have a maximum acceptable current density.
In particular, compared to inorganic light emitting diodes (LEDs), OLEDs are inferior because the inorganic LEDs can produce several hundred to several thousand times more light per unit area, primarily by operating at much higher current density. The organic architectural equivalent of an inorganic LED image bar exposes the photoreceptor at no more than a fraction of one page per minute. The exposure speed is slow because the amount of light emitted by an OLED necessary to expose the photoreceptor to the requisite degree to dispose a latent image on the photoreceptor occurs only over a significant amount of time. This slow exposure speed is not viable for commercial image bars.
Therefore, conventional OLEDs are not bright enough to use as light emitters for image forming engines that print at commercially required process speeds. This occurs both because of the less than 100% quantum efficiency of an OLED and because the necessary current density cannot be input into the OLED. If a sufficiently high amount of current could be input to the OLED, then a sufficiently high amount of light for image forming engines could be output from the OLED, regardless of the quantum efficiency. However, presently, the maximum current density input for conventional OLEDs is limited to approximately 20 mA/cm.sup.2. Exceeding the maximum current density causes the OLEDs to heat up and self-destruct. In other words, the OLEDs self-heat. This self-heating is a form of resistive heating. OLEDs are much more susceptible to damage as a result of self-heating than solid-state emitters because they use organic layers that melt or decompose at lower temperatures.
There have been a number of attempts to overcome this low quantum efficiency, by combining an OLED with electronic integration, as described in U.S. patent application Ser. No. 08/785,230 entitled "Integrating Xerographic Light Emitter Array" filed Jan. 17, 1997, and U.S. patent application Ser. No. 08/785,233 entitled "Integrating Xerographic Light Emitter Array with Grey Scale", filed Jan. 17, 1997, to Fork and Popovic, and by combining a thin film OLED with an edge emitter waveguide and electron injection layer, as described in U.S. patent application Ser. Nos. 08/771,089 entitled "Thin Film Organic Light Emitting Diode with Edge Emitter Waveguide filed Dec. 20, 1996 and 08/782,134 entitled "Thin Film Organic Light Emitting Diode with Edge Emitting Waveguide and Electron Injection Layer filed Jan. 13, 1997 to Wilson and Fork, each incorporated herein by reference in their entities. A similar structure is described in "Directed Beam Emission from Film Edge in Organic Electroluminescent Diode" by Hiramoto et al. Appl. Phys. Lett. 62(7) 666 (1973). However, as shown in FIG. 2, in the conventional edge emitting schemes, a light generating medium is in direct contact with a waveguide. This is problematic because the electrodes of the OLED directly induce an optical loss in the waveguide, which limits the effectiveness of the edge emitter.
As shown in FIG. 3, another conventional design is a thin film electro-luminescent (TFEL) device as described in "TFEL Edge Emitter Array for Optical Image Bar Applications", Kun et al. Proc. SID. 28(1), 81 (1987). A TFEL edge emitter utilizes thin film electroluminescent layers as a waveguide core substructures. These substructures, like analogous structures which can be made for OLEDs, have unacceptably large losses, on the order of 100 dB/cm. Therefore, conventional edge emitters that utilize organic materials do not generate sufficient light for commercially acceptable image forming applications.